Biological processes are routinely used for removing carbonaceous BOD and COD. These processes involve oxidation and reduction steps and biomass synthesis. In the last thirty years, the so-called advanced process modifications were developed to also remove phosphorus and nitrogen. These systems include various combinations of aerobic, fermentation, facultative anaerobic, anoxic, nitrification and denitrification zones. In nitrogen removal systems, the nitrogen containing species, usually ammonia, are biologically oxidized (nitrified) to nitrites and nitrates, following by a biological reduction (denitrification) of nitrites and nitrates to nitrogen and gaseous oxides of nitrogen. Phosphorus removal systems are conducted via, first, exposing the recycle sludge to facultative anaerobic conditions wherein acetic acid is formed and phosphorus is released, and, second, exposing biomass in the sludge to aerobic conditions wherein the so-called luxurious uptake of phosphorus is believed to occur. These systems are described in books "Phosphorus and Nitrogen Removal from Municipal Wastewater, Principles and Practice" Second Edition, Richard Sedlak, Editor, Lewis Publishers, 1991, "Biological and Chemical Systems for Nutrient Removal" A Special Publication, Prepared by the Task Force on Biological and Chemical Systems for Nutrient Removal, Movva Reddy, Chair, Water Environment Federation, 1998, and in patents, for example, U.S. Pat. Nos. 2,788,127, 2,875,151, 3,236,766, 3,964,998, 4,056,465, 4,162,153, 4,183,807, 4,183,808, 4,183,809, 4,183,810, 4,271,026, 4,488,967, 4,500,427, 4,500,429, 4,874,519, 4,867,883, 4,874,519, 4,917,805, 4,948,510, 4,999,111, 5,013,441, 5,022,993, 5,076,928, 5,076,929, 5,098,572, 5,160,043, 5,182,021, 5,213,681, 5,288,405, 5,480,548, 5,601,719, and 5,651,891.
The main problem with biological nutrient removal is that it is difficult to maintain balance between phosphorus and organics needed for the acetic acid production, and between nitrogen species and organics needed in the denitrification processes. Other problems may include low process efficiency, seasonal instabilities due to low temperatures, difficulties with pH control, complex systems, and complex operations. Additionally, the volume and costs of advanced biological reactors far exceed that of the conventional aerobic systems with complete nitrification. In biological phosphorus removal, there is a problem of phosphorus dissolution in the subsequent process steps, for example, during sludge treatment. Advanced biological systems usually require more energy than conventional systems. Large mass and volume of excess biosolids, also known as excess sludge or biomass, is generated.
Phosphorus can be easily removed with reagents, primarily, iron or aluminum ions in forms of insoluble phosphates. The process can be conducted in biological reactors so that little equipment is required. The stoichiometric iron and aluminum requirements are only 0.88 mg Fe and 0.28 mgAl per 1 mg P.sub.2 O.sub.4. The actual metal requirement is significantly greater. Chemical nitrogen removal involves difficult processes and is not practiced for low nitrogen concentrations in municipal and many industrial wastewater types. A co-precipitation of phosphorus and nitrogen (ammonia) as struvite (MgNH.sub.4 PO.sub.4) have also been discussed, but not practiced yet. The use of reagents contributes to the sludge mass. Increased mass of the excess sludge is cited as the argument against chemical phosphorus removal.
Anaerobic, aerobic, and coupled or combined anaerobic-aerobic biological systems are also used for treatment of dilute and concentrated industrial waste for organics and nutrients removal, as well as for removal and destruction of toxic and recalcitrant organics, heavy metals, sulfur containing compounds, and other applications. In these applications, biological processes may suffer problems similar to those experienced with advanced systems and also application specific problems. For example, in some applications, toxicity is a problem, in other applications, odorous constituents are emitted, while in others the process stability is difficult to control.